The invention relates to an apparatus for removing solid or liquid particles from a gas stream flowing in a helical path through a separating space bounded by two coaxial cylindrical walls, in which a radial unidirectional field is generated by means of electrodes associated with the cylindrical walls so that the particles move to the outside of the separating space by a co-operation of the centrifugal force and the electrostatic force.
An apparatus of this kind has been disclosed in British Pat. No. 487,547. However, this Patent does not indicate how the particles obtain the charge required for the electric separation.
In an apparatus for removing particles from a gas stream by means of an electric field, the field intensity has usually such a high value that a corona discharge occurs in the gas to be purified, whereby the gas is ionized and the particles to be removed are charged. The charged particles may now be removed from the gas stream by the attraction of the of the electrodes.
The use of a corona discharge has the disadvantage, however, that the electric circuits and their maintenance are relatively expensive and that not all particles may be removed from the gas stream. In fact, there is no effective separation if the resistivity of the particles is large, for instance larger than 10.sup.10 Ohm cm, because only a weak charge is obtained in this case, or if the particles have a low resistivity, for instance less than 10.sup.4 Ohm cm, because the change of polarity at the precipitation electrode occurs too rapidly in this case. Furthermore, it is necessary when a corona discharge is used to operate only slightly below the breakdown voltage, in order to obtain a reasonable efficiency. As a consequence there are frequent breakdowns causing interferences in radio and television receivers and a harmful generation of ozone. Also the voltage must frequently be adapted to the dirtying of the apparatus, whereby the danger of explosion occurs for certain kinds of dust.
In order to attain a corona discharge the field intensity in the separating space must lie above a critical value, which may be 10-20 kv/cm if the gas stream to be purified consists of air.
It has also been proposed to make use of a dipole effect in the electric purification of a gas stream. In this case the field intensity has such a low value that there is substantially no ionization in the gas stream and the separation is mainly based on the phenomenon that electric dipole charges are generated in the particles to be removed. This is of course always possible if the particles consist of an insulating material; however, a dipole charge may also be obtained in many cases for metallic particles, because they are often coated with an insulating oxide layer. For the dipole formation the field intensity must be smaller than the above-mentioned critical value if the gas stream consists of air. The proposal to use the dipole effect has not yet led to an industrial application, however.